Experimental characterization of an airfoil-based actuator using high-temperature shape memory alloys

This paper reports experimental results of an airfoil-based flap actuator that is actuated using high temperature Nickel-Titanium (NiTi) polycrystal and Copper-Aluminium-Nickel (CuAlNi) single crystal wires with a nominal diameter of 1.5 mm. The stress-free transformation temperatures of the commercially available NiTi wires are Mf = 53°C, Ms = 70°C , As = 95°C , Af = 110°C whereas those for the CuAlNi wires are Mf = 80°C ,Ms = 100.5°C, As = 104.5°C , Af = 117°C. Due to a significantly low electrical resistivity of the CuAlNi, the commonly used joule heating approach for thermal actuation is shelved for a heating coil approach. Uniaxial stress measurements, trailing edge flap deflections and temperature measurements are recorded during a typical heating and cooling cycle using a load cell in line with the SMA wire, a LVDT at the trailing edge tip and a thermocouple on the wire (outside the heating coil). It is seen that actuation by the CuAlNi (with a prestrain = 5.5%) leads to about a 50% higher tip deflection and about a 67% lower cooling time after actuation as compared to the corresponding values for NiTi (with a prestrain = 5.6%). The larger tip deflection is attributed to a higher strain recovery for the CuAlNi as compared to the NiTi during phase transformation whereas the lower actuation time is attributed, in part, to the narrow hysteresis in the stress-free transformation temperatures of the CuAlNi (~ 37°C) as compared to the NiTi (~ 57°C).